A Gremlin is a mythological creature similiar to a fairy or goblin which had their origins during the World Wars, in which they were said to cause havoc on aircraft - a trait which they have been continually given since their creation, though in time their destructive nature has come to include almost any type of technology.
It is said that Gremlins showed humanity how to create machinery, leading humanity to invention boom and industrial revolution. But humans showed these little creatures neither recognition nor acknowledgement. Angered by humans' ingratitude, Gremlins from that point on would sometimes cause sabotage to man-made machines, leading to breakdowns and accidents, sometimes with fatal results. Their acts of sabotage include caused engine failures, electrical malfunctions, communications shutdowns, bad landings, freak accidents, etc. and pretty much anything else that could possibly ever go wrong with an aircraft.
Although not popularly used until after World War II, the myth of the Gremlin seems to be older, the earliest example going back to the 1920s - it may of been a way for airmen to explain faults in aircraft or malfunctions, which were often seen as inexplicable and over time they became a part of the public imagination. Earliest literary reference to gremlins is in Pauline Gower's 1938 novel The ATA: Women with Wings, where Scotland is depicted as "gremlin country", a rugged territory where gremlins utilize scissors to cut wires of biplanes' engines.
Author named Roald Dahl is credited with getting the gremlins known outside the Royal Air Force. But while Roald is the person who should be acknowledged for getting gremlins known worldwide, it is 1984 film Gremlins that made Gremlins instilled into people's memories.
Physical descriptions of gremlins vary: They are described as being little humanoid elves, wearing red or green double-breasted frock coats, hats with feathers, and pointy shoes. The skin colors range anywhere between green, gold, pink, or red. Other depictions from different people give Gremlins more sinister appearance; they look feral, have hairy bodies, large pointy ears, glowing red eyes, and horns. More reports describe Gremlins as having nude grey skins, somewhat reptilian in appearance, and have big mouths with sharp teeth. There had been reports that say Gremlins look like jackrabbits, bull terriers, or some combination of both. In some cases, they are reported to be composed of mists or smokes. Some accounts mention Gremlins having merfolk features such as webbed hands and feet, fins. Some reports say Gremlins possess bat-like wings. Size descriptions also vary: Gremlins are said to be anywhere between a mere 6 inches tall all the way up to three feet in height. In some cases they are said to have big feet with suctions on their soles or even leather shoes with hooks, both of which would enable Gremlins to walk about or to hang upside down on the outside of aircraft.
Gremlins during WW1 and WW2, are thought, at one point, to have sympathies towards Axis and Central Powers, but then Axis and Central Powers had similar and equally inexplicable mechanical problems in their military machines, hinting that Gremlins are equal opportunity tricksters, apparently taking no sides in the conflict, and acting out their mischief from their own self-interest.
Gremlins have become a staple of fiction and inspired many mischievious and/or malicious beings throughout the ages - the most famous of which are arguably the titular monsters of the comedy-horror Gremlins (though these monsters bared little in common with the folkloric creatures).
The origin of the modern term 'gremlin' is disputed, but is often said to derive from the Old English word greme, which means to vex or annoy. Gremlins probably have roots in the old folklore of goblins and fairies.